924Board.org

[staticsan]

My '82 NA is having issues. It's been difficult to start for months: one cylinder doesn't run properly for several minutes but once it takes, it's been good.

Maybe a month or two ago, I noticed it started stumbling sometimes at moderately high revs (4k-ish). Couldn't reproduce it reliably and then it went away. In the last few weeks, though, the stumbling has become more pronounced and much repeatable: it happens under load, no matter the revs.

Now it's become hard to start when warm with the same not-running-on-all-cylinders as when it's cold. And in the last few days, after an hour or so of driving, it will go rough again in idle.

I've been thinking a lot about it and suspect one injector is getting blocked and injector cleaner is no longer working. (Perhaps the repeated application of injector cleaner every three or four tanks of fuel has dislodged some serious gunk somewhere else.) Does this sound like a plausible explanation?

It is unfortunately difficult to work on in my garage mostly due to poor light, which means taking it somewhere first for serious work. With my limited resources, I was going to try swapping out the injectors I bought a few years ago for the ones it came with. But I'm a bit worried that will just put the problem off again for a while rather than solving anything.

Wade.
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'82 British NA - Which I think I've been very lucky with!

[ideola]

Fuel delivery definitely sounds like a possible culprit, and starting with a check of injector spray pattern is a good first step. While doing this, you should also be mindful that the problem could be upstream of the injectors, particularly at the fuel distributor. So if you find that one injector is spraying too much or too little, do a quick swap with an injector that appears to be working correctly. If the problem follows the injector, the injector is bad; otherwise, you have a problem at the fuel distributor.

The injectors are manually operated based on fuel pressure, and when the internal springs fail, the injector is done and needs to be replaced.

The fuel distributor can be a harder problem to diagnose, as it could be a blocked port or line (too little flow) or worn-out o-ring (too much flow). While some here have attempted FD rebuilds on their own, this is not a job I would recommend for a DIY. If the FD turns out to be the culprit, I would suggest having it rebuilt by a qualified specialist.

Of course, there may be other fuel delivery problems, so you should consider the full battery of CIS tests (system pressure cold, control pressure cold, system pressure hot, control pressure hot, and leakdown).

Stumbling can also be caused by bad spark issues...bad plugs, bad leads, bad dizzy cap, bad rotor, bad coil, bad ignitor. So if the fuel delivery issues check out, you should move on to spark diagnosis. The key is to be methodical about it and to record the results of each test along the way as you eliminate variables.

HTH.
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erstwhile owner of just about every 924 variant ever made

[Ozzie]

What he said,
had a misfire, tracked down to a bad injector.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Ian89C4]

I was having a "stumbling" issue at high RPMs and high heat (race car) and went all the way from replacing fuel pump, to replacing the fuel dizzy (more for a richening issue under load than anything), and finally got to the fuel injectors. I thought that they would be the least of the problems, but the pic will show you that an engine may be running, but there are issues that are not seen (don;t mind the color, its just condensation on the outside of the jars). Check the injectors. Don't forget to check the cold start valve, it may be continuously running and fouling plugs.



If they are good than I would check coil, ballast resistor (I did not look to see what year your 24 is, later ones don't use one), plugs, wires, dizzy cap, dizzy rotor, check the connection with your ignition relay (in the front left of the engine bay on the wheel well).

Hope all works out.

Cheers,
Ian
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Ian Edgerly
North Carolina

Current:
1981 924 SCCA/ChampCar Weissach Race Car ("Serenity")
1987 924S ChampCar Endurance Car ("Der Weg")
1980 924 Euro ("Wahala")

[ideola]

Ian, that is such a perfect illustration. Too much fuel on #4, too little on #3.
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erstwhile owner of just about every 924 variant ever made

[staticsan]

Wow.

I had a chat to Dad and we came to the conclusion that we really need someone with a bit more experience handling live fuel systems than either of us have to at least supervise. This is the same reason we've never done a pressure test. Carburettors he understands. I can find friends who know electronic systems. But the K-Jetronic is in the knowledge hole.

My mechanic is back from holidays next week so I'll give him a call. He has worked on K-Jetronic systems for years.

I should have the ability to do this, but I really don't.

Wade.
_________________
'82 British NA - Which I think I've been very lucky with!

[Ozzie]

These are not a magical art, follow the Haynes for testing and you should not have any problem.
without complicated equipment (I used 4 empty drink bottles) I measured the flow from the injectors by bypassing the pump relay and lifting the flap in the CIS dissy. Checked the spray pattern too.
given a set time, they should be equal.
If not, replace the injectors. I use Dr Injector on ebay who guarantees his parts.

Fuel flow should be a set volume over a set time. I used a measuring jug for this. Again, nothing complicated. (wife wasn't happy though)
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance